Last week marked only the second time in his life that Thad Cochran did not win an election outright.

The previous instance was 18 years ago this month, when he was defeated for Senate majority leader by Mississippi’s other Republican senator at the time, Trent Lott. That contest foreshadowed as clearly as anything the dire political predicament Cochran finds himself in now — just two weeks from a GOP primary runoff where state Sen. Chris McDaniel seems to have most everything going his way .

The outcome will decide more than whether Cochran is denied a seventh term. His defeat would guarantee that, come 2015, the chamber would have just two members who knew life in the Senate before Ronald Reagan was president. A McDaniel victory would allow the tea party movement to portray its confrontational style of conservatism as alive and well in the top tier of American politics.

And the only primary defeat of an incumbent senator this year would bring down the curtain on a fading era at the Capitol. Cochran was already an anomaly because he never wavered from the view that being urbane and soft-spoken in public, and collegial and collaborative behind the scenes, was the surest route to institutional success and job satisfaction. But that approach, of course, has almost entirely fallen out of fashion on both sides of the aisle and on both sides of the Capitol — supplanted by a pathway in which partisan bombast and reflexive combativeness are rewarded while cordiality and thoughtfulness are ridiculed.

This shift in the congressional culture was given one of its first high-profile Senate displays in June 1996, when Bob Dole unexpectedly gave up the GOP floor leader’s job (along with his Kansas seat) to focus on his challenge to President Bill Clinton’s re-election. The race for a successor came down to a contest between Dole’s top deputies. Lott, then the energetic and publicity-loving majority whip, portrayed himself as the candidate of the harder-charging younger breed of GOP conservatives and promised them a more aggressive style of leadership. Cochran, who brought a quiet courtliness to the chairmanship of the Republican Conference, marketed himself as an institutionalist who would combat the rise of shrill politics and help rebuild public support of Congress by sticking with the old-guard value of compromise over conflict.

In that prelude to so many subsequent Republican showdowns between the insurgents and the establishment, the outcome wasn’t even close: Lott was backed by 44 colleagues in the secret ballot. Cochran’s supporters numbered only eight.

Instead, diminished turnout for the second round of voting should favor the organizational energy of the small-government groups promoting the 41-year-old McDaniel. He finished ahead of the senator by about 1,400 votes last week, but was just shy of the 50 percent needed to advance without a runoff to the general election, where the Democratic nominee is centrist former Rep. Travis Childers . A main argument for Cochran in the coming days will be that if McDaniel is the nominee, his unpredictable and outspoken conservatism would create an opening for Democrats to pick up a Senate seat in one of the reddest states in the country.

If Cochran loses on June 24, it would mark an extraordinary comedown for the first GOP senator elected from Mississippi since Reconstruction — a 1978 victory that made him an avatar for the rise of Republicanism across the South and climaxed a rapid succession of personal triumphs.

At the University of Mississippi he won the vice presidency of the student body, was picked by his fraternity brothers as president of Pi Kappa Alpha and was an Ole Miss cheerleader, an elected post that’s launched many a political career in the state. He was made partner at a prestigious Jackson firm three years out of law school and four years after that, in 1972, won the first of his three terms in the House.

After his initial election to the Senate, his subsequent races were such cakewalks that Democrats did not even field challengers in 1990 or 2002. He won by 23 points in 2008.

And, though bumped off the leadership ladder, Cochran was able to leverage his seniority to gain other positions of power. He chaired the Agriculture and Appropriations committees for two years each in the past decade, and for the past six years he has held the top GOP seat on the Defense Appropriations Subcommittee, which decides half of all discretionary spending.

His reputation as a big-government defender out of step with current GOP beliefs is based mainly on his years of unabashed and successful efforts to earmark federal money for his state. He’s won billions beyond what was initially estimated for Hurricane Katrina recovery along the Gulf Coast a decade ago, millions more than the Navy has sought for work at the big shipyard in Pascagoula over the years, and even captured $131,000 in 2008 to create a national music repository at Ole Miss.

A look at his overall voting record, though, suggests Cochran has become more conservative in the past two decades — at about the same pace as the overall Senate GOP. In 1995, the year before he ran for majority leader as the establishment’s candidate, there were 10 Republican senators to Cochran’s left — as measured by how often they voted the way Clinton wanted. And last year, the number was identical: 10 of his GOP colleagues backed President Barack Obama more frequently than Cochran’s 49 percent.

That Cochran has effected such an ideological shift without drawing much attention is indicative of his genteel, even inscrutable ways. And that style, in turn, has much to do with why his career is now in so much jeopardy.